Motors are employed in a multitude of situations requiring various orientations. Among other uses, motors can be used in fans or drive a toy car. A fan driven by a brushless DC motor, for example, has a movable portion (i.e., a rotor) rotatably coupled with a stationary portion (i.e., the stator). The rotor typically has an impeller (or propeller) coupled to a shaft that is rotatably secured to a bearing within the stator. To rotate the rotor, the stator produces a rotating magnetic field that, when interacting with a magnet in the rotor, causes the rotor to rotate about an axis defined by the shaft.
Motors can use different types of bearings to rotatably secure the shaft to the stator. For example, some motors use a sleeve bearing, which generally is a highly lubricated, porous sleeve that maintains the radial position of the rotor within the motor. Lubrication of the porous walls of the sleeve bearing should minimize friction that arises between the shaft and sleeve walls, thus permitting the rotor to more freely rotate.
The hub of the rotor (in conjunction with other components) commonly maintains the axial position of the rotor. More particularly, some fans have a thrust washer captured between the rotor hub and sleeve bearing. One function of the thrust washer is to act as a loose lid on the sleeve bearing, thus retaining (to some extent) lubricant within the porous walls of the sleeve bearing. During operation, the hub rubs against the thrust washer that in turn rubs against the top of the sleeve bearing. If this friction is not minimized, fan efficiency is reduced, which leads to the need for larger fan motors and additional power consumption. Both consequences are adverse to the use of fans in cooling increasingly smaller and hotter operating electrical components and equipment.